1. Field of the Invention
The present invention relates to a terahertz wave spectrometer for performing spectroscopic measurements by using a terahertz wave. The terahertz wave is an electromagnetic wave having a frequency of around 1 THz (terahertz).
2. Description of Related Art
An electromagnetic-wave frequency range of around 1 THz (terahertz) is located on the boundary between an optical wave and a radio wave, and is called a terahertz range. More specifically, the terahertz range is defined as a frequency range that includes frequencies of about 100 GHz to 10 THz. The terahertz range can sometimes be defined as a wider frequency range that includes the range of about 100 GHz to 10 THz and further includes neighboring lower- and higher-frequency ranges. For example, the terahertz wave can effectively be used in spectroscopic processes for an infrared region and in imaging processes for the infrared region.
In comparison with other frequency ranges, developments of generators and detectors for this frequency range advance relatively slowly. Many technological and other problems have not yet been solved to apply the generators and detectors in practical uses. It is necessary to develop terahertz wave generators (optical sources) and terahertz wave detectors so that they will be small and easy to handle, in order to realize, in an industrial field, a spectrometer that detects and quantitatively measures the characteristics of a sample by using terahertz wave.
There are recently being developed terahertz wave generators (optical sources) and terahertz wave detectors that employ optical switching devices or electro-optic (EO) crystals. Though it is difficult to generate electromagnetic wave at the terahertz-range frequency by using an electric-circuit oscillator, it is possible to generate and detect electromagnetic wave at the terahertz-range frequency by modulating an electric current, or the like, using a pulse-shaped light.
In order to attain the spectroscopic measurement by using terahertz wave, one method has been proposed to detect terahertz wave and to measure directly the intensity of the temporal waveform of the detected terahertz wave. However, this method provides no limitation or no selection onto the respective frequency components of the terahertz wave. Accordingly, even if the sample presents some special characteristic with respect to a specific frequency range, it is impossible to perform measurement at the specific frequency range only. Thus, only a limited amount of information can be obtained from the spectroscopic process.
A method for performing measurements with frequency selection has been proposed by xe2x80x9cTerahertz Electromagnetic Wave: Generation and Applicationsxe2x80x9d by Sakai et al. Laser Review, Vol. 26, No.7, pp.515-521 (1998). During the measurement process with frequency selection, a detector performs a sampling measurement. A temporal waveform of the terahertz wave is determined based on the sampling-measurement result. Next, the obtained temporal waveform is subjected to fast Fourier transform (FFT), and the resultant amplitude spectrum is evaluated. In this device, the terahertz wave is scanned only once in a forward direction by a variable optical delay device, thereby sampling the terahertz wave.
Japanese Patent Unexamined Application Publication Nos. 8-320254 and 10-153547 disclose imaging systems that obtain spectroscopic information by using terahertz wave and by using an analog-to-digital converter and a digital signal processor (DSP). According to the methods disclosed by these publications, the DSP retrieves frequency-related information from time domain data by recognizing the characteristic shape of the terahertz wave.
According to the above-described conventional methods, the temporal waveform is first measured, and then frequency-related information, such as an amplitude spectrum, is determined by a computer thereafter. It is therefore impossible to attain a real-time measurement. Additionally, the entire device for attaining those methods has a complicated structure. Especially, the analog-to-digital converter and the DSP are employed to attain the methods disclosed by the publication Nos. 8-320254 and 10-153547. Accordingly, the device, such as a two-dimensional array for performing an imaging operation, becomes complicated and expensive,
It is an objective of the present invention to solve the above-described problems, and to provide a terahertz wave spectrometer, which can perform spectroscopic measurement in real time and whose device structure is simplified.
In order to overcome the above-described problem, the present invention provides a terahertz wave spectrometer for performing spectroscopic measurement by using terahertz wave, comprising: a predetermined excitation light optical system guiding an excitation light; a terahertz wave generator generating terahertz wave by using the excitation light guided by the predetermined excitation light optical system; a terahertz wave optical system guiding the terahertz wave generated by the terahertz wave generator to a sample for spectroscopic measurement, and further guiding the terahertz wave which has been affected by the sample; a predetermined probe light optical system guiding a probe light that is in synchronization with the excitation light; a terahertz wave detector detecting, using the probe light guided by the predetermined probe light optical system, the terahertz wave that is affected by the sample and that is guided by the terahertz wave optical system, and outputting a detection signal; optical delay vibrating means provided in either one of the excitation light optical system and the probe light optical system, the optical delay vibrating means vibrating, at a predetermined vibration frequency, the length of the optical path of the corresponding one of the excitation light and the probe light, thereby periodically vibrating the irradiation timing of the corresponding one of the excitation light and the probe light onto a corresponding one of the terahertz wave generator and the terahertz wave detector; and spectroscopic processing means performing spectroscopic measurement on the sample based on the detection signal obtained by the terahertz wave detector, the spectroscopic processing means including frequency analyzing means performing frequency analysis on the detection signal that periodically changes in accordance with the vibration frequency, the frequency analyzing means performing the frequency analysis of the detection signal by performing a frequency domain measurement, the frequency-analysis result obtained by the frequency analyzing means indicating frequency-analysis information on the terahertz wave that has been affected by the sample, thereby indicating the spectroscopic information of the sample.
In the terahertz wave spectrometer having the above-described structure, the irradiation timing of the probe light onto the terahertz wave detector is synchronized with respect to the irradiation timing of the excitation light onto the terahertz wave generator, while vibrating or oscillating the difference between the probe light irradiation timing and the excitation light irradiation timing. More specifically, the terahertz wave spectrometer is set up so that the terahertz wave emitted from the terahertz wave generator is transmitted through the predetermined terahertz wave optical system, is affected by the sample upon passing through or reflecting off the sample, for example, and then falls incident on the terahertz wave detector, which in turn detects the terahertz wave by using the probe light. The terahertz wave spectrometer is set up to vibrate or oscillate the irradiation timing of the terahertz wave on the terahertz wave detector and the detection timing of the terahertz wave by the probe light at the terahertz wave detector.
In the case where the optical delay vibrating means is provided in the probe light optical system, the optical delay vibrating means preferably includes a portion constructed to change the length of the optical path of the probe light. By driving this portion at a predetermined frequency, the detection timing of the terahertz wave periodically vibrates or oscillates. It is noted that by changing the detection timing, it is possible to scan the temporal waveform of the terahertz wave. According to the present invention, because the detection timing is changed in a vibrating or oscillating manner, the temporal waveform of the detection signal has a time scale converted from the time scale of the temporal waveform of the terahertz wave. The temporal waveform of the detection signal has a shape converted from the shape of the temporal waveform of the terahertz wave in a predetermined rule. For example, the shape of the temporal waveform of the detection signal may be exactly the same as the shape of the temporal waveform of the terahertz wave. Or, the shape of the temporal waveform of the detection signal may be the quasi-same, or similar, with the shape of the temporal waveform of the terahertz wave. Or, the shape of the temporal waveform of the detection signal may correspond to the shape of the temporal waveform of the terahertz wave by a predetermined rule. The frequency spectrum of the detection signal has a frequency scale converted from the frequency scale of the frequency spectrum of the terahertz wave. The frequency spectrum of the detection signal has a shape converted from the shape of the frequency spectrum of the terahertz wave in a predetermined rule. For example, the shape of the frequency spectrum of the detection signal may be exactly the same as the shape of the frequency spectrum of the terahertz wave. Or, the shape of the frequency spectrum of the detection signal may be the quasi-same, or similar, with the shape of the frequency spectrum of the terahertz wave. Or, the shape of the frequency spectrum of the detection signal may correspond to the shape of the frequency spectrum of the terahertz wave by a predetermined rule. By vibrating or oscillating the detection timing, it is therefore possible to convert the frequency scale of the terahertz (THz) order into a desired frequency scale, such as a kilohertz (kHz) order, for example.
Alternatively, the optical delay vibrating means may be provided to the excitation light optical system, rather than to the probe light optical system, thereby vibrating the terahertz wave generating timings. Also in this case, it is possible to attain scale conversion onto the temporal waveform and onto the frequency spectrum.
According to the present invention, by performing frequency domain measurement onto the detection signal by the frequency analyzing means, it is possible to measure the frequency of the detection signal directly. The thus obtained frequency information of the detection signal is frequency-scale converted frequency information of the terahertz wave, and therefore indicates the spectroscopic information of the sample. By measuring the frequency of the detection signal directly in this way, it is possible to perform spectroscopic measurement of the sample.
In this way, according to the present invention, the frequency of the detection signal is measured directly. Accordingly, contrary to the conventional technology that performs time domain measurement, it becomes unnecessary to perform fast Fourier Transform calculation or the like. It therefore becomes possible to perform real-time measurement. Because it is unnecessary to perform the fast Fourier Transform calculation or the like, it is possible to perform frequency analysis on the terahertz wave by using the simplified data processing method and by using the simplified device configuration. Accordingly, it is possible to realize a spectrometer which can attain a real-time spectroscopic measurement, whose device structure is simplified, and which can be made less costly. Simplifying the structure allows the device to be assembled into an integrated circuit configuration.
The optical path length (and therefore the detection timing) may be vibrated or oscillated in a triangular waveform or a sawtooth waveform, in order to linearly scale-convert the temporal waveform and the frequency spectrum of the terahertz waveform into those of the detection signal while exactly maintaining the shapes of the temporal waveform and the frequency spectrum. The optical path length (and therefore the detection timing) may be vibrated or oscillated in a sinusoidal waveform, in order to nonlinearly scale-convert the temporal waveform and the frequency spectrum of the terahertz waveform into those of the detection signal while converting the shapes of the temporal waveform and the frequency spectrum into the quasi-same shapes. The vibration waveform may be optionally selected so that the detection signal will correspond to the original terahertz wave with a conversion rule that is proper for the device configuration and for the measuring conditions.
The frequency analyzing means may preferably include a spectrum analyzer producing a frequency spectrum by performing a frequency analysis on the detection signal. In this case, the produced frequency spectrum is indicative of the spectroscopic information of the sample. Measurements can be attained under various conditions. For example, measurements can be attained by narrowing the measurement frequency range of the spectrum analyzer, or by limiting the measurement frequency range to a specific frequency.
The frequency analyzing means may include a band pass filter selecting a predetermined frequency component from the detection signal. In this case, the detection signal at the selected frequency component is indicative of the spectroscopic information of the sample. The device structure can be further simplified by thus using the band-pass filter.
The band pass filter may include a plurality of band pass filters for selecting frequency components different from one another, and the spectroscopic processing means may further include correlation analyzing means determining a correlation between the plurality of frequency components selected by the plurality of band pass filters. By using the correlation, such as a difference, between the plural frequency components, it is possible to obtain a greater amount of spectroscopic information, and also to enhance the efficiency in the spectroscopic measurement.
The spectroscopic processing means may further include frequency setting/changing means controlling the optical delay vibrating means and changing or setting the value of the vibration frequency, at which the optical delay vibrating means vibrates the length of the optical path for the corresponding one of the excitation light and the probe light, the frequency analyzing means performing the frequency analysis based on the thus changed or set vibration frequency. It is possible to freely control the condition of the time/frequency scale conversion from the terahertz wave into the detection signal, thereby freely changing and setting the frequency range to be detected.
In order to construct the terahertz wave generator or the terahertz wave detector as suitable for generation or detection of the terahertz wave, for example, at least one of the terahertz wave generator and the terahertz wave detector may be constructed from an optical switching device. Alternatively, at least one of the terahertz wave generator and the terahertz wave detector may be constructed from an electro-optic crystal.
The terahertz wave spectrometer may further comprise sample moving means moving the sample two-dimensionally, thereby causing the spectroscopic processing means to perform two-dimensional spectroscopic measurement on the sample.
Alternatively, the terahertz wave detector may preferably be constructed from a two-dimensional detector for performing a two-dimensional spectroscopic measurement on the sample under investigation. That is, the terahertz wave detector may be constructed from a two-dimensional detector, in which a plurality of terahertz wave detecting portions are arranged two-dimensionally, the spectroscopic processing means including a plurality of frequency analyzing means, the plural terahertz wave detecting portions being connected to the plural frequency analyzing means, respectively, each frequency analyzing means performing frequency analysis on a detection signal obtained by the corresponding terahertz wave detecting portion, thereby attaining two-dimensional spectroscopic measurement on the sample. With this structure, it is possible to attain measurement, such as a two-dimensional imaging, on the sample in real time. This is effective to the spectroscopic measurement to measure distribution of components in the sample.
The excitation light optical system may include an optical chopper controlling on and off of the excitation light. In this case, it is possible to enhance the signal-to-noise ratio of the measurement by reducing the influences from the 1/f noise, which is generated by the source for the excitation light and the probe light.
The spectroscopic processing means may further include analyzing means determining frequency analysis of the terahertz wave, which is affected by the sample and which indicates the spectroscopic information of the sample, based on the frequency-analysis result of the detection signal obtained by the frequency analyzing means. The analyzing means performs processings, such as converting the frequency scale of the frequency-analysis result of the detection signal back to the frequency scale of the original terahertz wave, thereby obtaining frequency-analysis information of the terahertz wave, that is, the spectroscopic information of the sample.
In this way, in the terahertz wave spectrometer of the present invention, the optical delay vibrating means vibrates the irradiation timings of the probe light or the excitation light at the predetermined frequency, thereby causing the detection signal obtained by the terahertz wave detector to have a signal waveform whose time scale is converted from the time scale of the temporal waveform of the terahertz wave. Additionally, the frequency analyzing means, which employs the spectrum analyzer, the band-pass filter, or the like, is applied to the detection signal which has such a signal waveform. Frequency analysis is performed on the detection signal whose frequency scale is converted from the frequency scale of the terahertz wave. Spectroscopic measurement is attained based on the analyzed result output. Accordingly, it is possible to realize a terahertz wave spectrometer which can perform real-time spectroscopic measurement of terahertz wave, including the two-dimensional imaging, whose device structure is simplified, which can be made less costly, and which can be assembled into an integrated circuit structure. It is also possible to enhance the signal-to-noise ratio of the spectroscopic measurement. It is therefore possible to attain a more accurate measurement, and to reduce the period of time required to attain the measurement. The terahertz wave spectrometer of the present invention can enable the terahertz wave spectroscopy to be applied to a wider area of practical use.
The frequency analyzing means may preferably detect a desired frequency component of the detection signal by performing the frequency-domain measurement. The frequency analyzing means may be constructed from a spectrum analyzer. The spectrum analyzer may be set to a zero span mode. Or, the frequency analyzing means may include a band pass filter selecting the desired frequency component, and the spectroscopic processing means may further include frequency setting/changing means controlling the optical delay vibrating means and changing or setting the value of the vibrating frequency, at which the optical delay vibrating means vibrates the length of the optical path of the corresponding one of the excitation light and the probe light, to a value that corresponds to a frequency value of the desired frequency component to be selected by the band pass filter. The terahertz wave detector may be constructed from a two-dimensional detector, in which a plurality of terahertz wave detecting portions are arranged two-dimensionally, the spectroscopic processing means including a plurality of band pass filters, the plural terahertz wave detecting portions being connected to the plural band pass filters, respectively, each band pass filter performing frequency-domain measurement on a detection signal obtained by the corresponding terahertz wave detecting portion to select the desired frequency component, thereby attaining two-dimensional spectroscopic measurement on the sample. Additionally, the excitation light optical system may include an optical chopper controlling on and off of the excitation light at a predetermined driving frequency, the frequency analyzing means detecting, by performing frequency-domain measurement, a frequency component of the detection signal that is determined with respect to the predetermined driving frequency. The frequency analyzing means may include a spectrum analyzer. Or, the frequency analyzing means may include a band pass filter.
The terahertz wave spectrometer according to the present invention can be broadly used in a variety of measurements for measuring the kind, the amount, and the distribution of material in a sample by allowing the sample to affect terahertz wave by causing the terahertz wave to pass through the sample or to reflect off the sample, for example, and then analyzing the frequency of the terahertz wave affected by the sample. It is possible to use any of gases, liquids, and solids as the sample under investigation. It is therefore possible to use the terahertz wave spectrometer according to the present invention broadly in measurements of many kinds of samples, such as of air pollution, exhausted gas, waters semiconductor, and dielectric material.